The present invention relates in general to a method for acoustic well logging and, more particularly, to a method for detecting the presence of hydraulically induced fractures behind well casing and determining their azimuthal orientation.
It has long been known to acoustically log open wellbores to determine the velocities of compression ("P") waves and shear ("S") waves traveling through rock formations located in the wellbore region and tube waves ("T") traveling along the wellbore interface. Logging devices have been used for this purpose which normally comprise a sound source (transmitter) and one or more receivers disposed at pre-selected distances from the sound source.
By timing the travel of compression waves, shear waves, and/or tube waves between the transmitter and each receiver, it is normally possible to determine the nature of surrounding rock formations including natural fracture identification. For descriptions of various logging techniques for collecting and analyzing compression wave, shear wave, tube wave, and secondary wave data, please refer to U.S. Pat. Nos. 3,333,238 (Caldwell); 3,356,177 (Loren); 3,362,011 (Zemanek, Jr.); Reissue No. 24,446 (Summers); 4,383,308 (Caldwell) and 4,715,019 (Medlin et al); and to "The Correlation of Tube Wave Events With Open Fractures In Fluid-Filled Boreholes" by Huang and Hunter in Geological Survey of Canada, pgs. 336-376, 1981.
In each of the foregoing references, the acoustic waves are generated in the formation in response to an acoustic energy transmission from within an open wellbore. However, the teachings of such references are not applicable to the identification of induced fractures in formations which are traversed by well casing, that is, well pipe cement bonded to the formation. An acoustic transmitter within the cased well generates tube waves which travel along the casing surface. These casing tube waves are the predominant mode of wellbore excitation at low frequencies. Their amplitudes are orders of magnitude greater than those of compressional or shear waves generated in the casing or formation. In cemented intervals the casing tube waves can be sensitive to changing conditions behing the casing.
It is therefore a specific objective of the present invention to provide for a method of logging a cased well to detect hydraulically induced fractures in the formations surrounding the cased well and to specify their azimuthal orientation.